456 Mr Growther, On the Transmission of ^-rays. 



entirely different character to those emerging from aluminium 

 when studied in a second magnetic field. While the latter are 

 gradually reduced in velocity without any appreciable dispersion 

 of their velocities about the mean, the former seem to possess 

 a very considerable range of velocities. It may be noted that 

 McClelland* has shown that the return /3-radiation from any 

 radiator may be divided into two parts, which differ in their 

 penetrating power and in their law of distribution. One has 

 practicall}'' the same penetrating power as the primary /3-ra.js, 

 and is probably purely scattered radiation; the second type is 

 less penetrating and is probably a true secondary radiation. The 

 former is the predominating factor in elements of low atomic 

 weight such as aluminium, while the latter predominates in 

 elements of high atomic weight such as lead or platinum. 



It seems, therefore, that the first stage in the absorption of a 

 beam of homogeneous /S-rays by platinum is their transformation 

 into secondary radiation (during the initial steeper portion of 

 the curve). The absorption of these secondary rays in the further 

 sheets of platinum then takes place according to an exponential 

 law. The rapidity of this conversion, which seems to be complete 

 in O'OOl cm. of platinum, is at first somewhat startling. It is, 

 however, quite in accordance with previous experiments f on the 

 scattering of the yS-rays from uranium, which may be regarded 

 as showing that the distribution of the /S-rays in a sheet of gold 

 reaches its final form in a thickness of little more than 0"0002 cm. 



It is interesting to notice that Sadler]: has very recently 

 shown that the absorption of the secondary corpuscular radiation 

 emitted from different radiators under the action of homogeneous 

 secondary Rontgen rays is also absorbed according to an expo- 

 nential law. 



We are thus led to the following result. When /3-rays are 

 emitted by any substance, whether due to its own radio-active pro- 

 perties, or excited by external radiation of a single definite type, 

 the absorption of the rays emitted follows an exponential law. 

 On the other hand the absorption of a homogeneous beam of 

 yQ-rays by a substance such as aluminium which does not emit 

 any large amount of true secondary radiation of its own, follows 

 a law the precise nature of which remains to be determined, but 

 which is certainly not exponential. 



If this is so, it follows, as Wilson has already suggested, that 

 the rays from a single radio-active substance are absorbed accord- 

 ing to an exponential law by virtue of some special distribution 

 of velocities in the emergent beam. The results obtained in the 



* J. A. McClelland, Proc. Roy. Soc. A, Vol. lxxx. p. 501. 

 t J. A. Crowther, Proc. Roy. Soc. A, Vol. lxxx. p. 187. 

 :J: C. A. Sadler, Phil. Mag. March 1910. 



